Support CUDA streams

PyTorch includes [CUDA streams](https://pytorch.org/docs/stable/notes/cuda.html#cuda-streams), which let multiple GPU requests run in parallel.

However it appears that TorchSharp does not support CUDA streams. I searched the codebase and can't find anything like PyTorch's torch.cuda.Stream class, or C# wrappers for e.g. the wait_stream(), default_stream() and record_stream() methods.